1. Field of the Invention
The present invention relates to an optical isolator, especially an optical wave guide capable of being integrated on a substrate together with a semiconductor laser and an optical wave guide.
2. Related Art Statement
In the optical communication system, an optical isolator is used to prevent reflected light and scattered light in the end face of the optical fiber from returning to a light source side. An optical isolator using a rotation of polarization plane according to the optical-magnet material, has been put to practical use as an optical isolator now. For example, the optical isolator of Farady rotation type is constituted by a polarizer, a Faraday rotator of optically transparent material and an analyzer, so that polarized components corresponding to the plane of polarization of polarizer out of advanced circular polarization of light to the forward direction, pass through the polarizer, the plane of polarization rotates by Faraday rotator by 45.degree., and emanate by passing the analyzer which is inclined at the polarizer by 45.degree.. On the other hand, the return light propagated in the direction opposite to the forward direction is obstructed with a polarizer, since after passing the analyzer, the plane of polarization of the return light receives the rotation of 45.degree. by the Faraday rotator and returns to the polarizer. The polarized beam splitter and the double refraction prism are utilized as polarizer and analyzer used by conventional optical waveguide isolator.
The above well known optical waveguide isolator was not able to be manufactured with a semiconductor optical element such as semiconductor lasers and optical modulators as one body, since the well known optical waveguide isolator does not have the semiconductor device structure. Therefore, the well known optical waveguide isolator must be made as another discrete structure with the substrate of the optical circuit to which the semiconductor device such as semiconductor lasers is integrated, so that the manufacturing step becomes complicated and the manufacturing cost becomes expensive, too. Particularly, since the phase matching is necessary, and a precise processing is necessary, so that the manufacturing process becomes complex. On the other hand, if the optical waveguide isolator can be formed on the substrate by using the same semiconductor manufacturing technology as semiconductor devices such as semiconductor lasers and photo diodes, it can manufactured with precise manufacturing step and thus the manufacturing cost can be made cheap greatly.